The mutagenic and carcinogenic activities of previously untested agents were investigated. Salmonella TA100 was used to determine the mutagenic activities of alkyl halides. The ability of alkyl halides to produce lung adenomas in strain A mice was also studied. In general a linear correlation was observed between mutagenicity and carcinogenicity. The antagonism between the essential divalent metals calcium and magnesium and the divalent metal carcinogens, lead, nickel and cadmium are under investigation in metabolic and in carcinogenicity studies. Magnesium and calcium completely prevented lung adenoma production in strain A mice by nickel and lead acetates.